Best Development Practices

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1 Town of Winchendon E. Himlan, Mass. Watershed Coalition Berkshire Design Group Best Development Practices Version 1, March 2010 Guidebook

2 Table of Contents INTRODUCTION... 1 I. CHECKLIST FOR DESIGNERS... 2 II. STORMWATER MANAGEMENT... 7 a. Overview and Policies... 7 b. Summary of Practices... 9 c. Discussion of Practices Vegetated Water Quality Swales (Dry & Wet) Vegetated Filter Strips Constructed Wetlands Bioretention Cells (Rain Gardens) Porous Pavement Tree Box Filter Green Roofs Wet Basins Dry Detention Basins Deep Sump Catch Basin III. EROSION AND SEDIMENTATION CONTROL a. Site Planning b. Construction Period Impacts Soil Stabilization Sediment Retention Perimeter Protection Other Practices IV. LANDSCAPE DESIGN a. Water-Sensitive Landscaping b. Plant Species Native Species Salt Tolerant Species Urban Tolerant Species Species for Erosion Control Wetland Species Moist Tolerant Species Drought Tolerant Species Habitat Creating Species Invasive Species V. SITE PLANNING ENDNOTES Winchendon Best Development Practices Guidebook

3 Introduction The Winchendon Best Development Practices Guidebook (BDP Guidebook) is a set of guidelines for developers, designers and project reviewers intended to improve the quality of development in Winchendon. The Guidebook describes the preferred design and construction practices in Winchendon for stormwater management, erosion and sedimentation control, landscape design, and site planning. None of the practices in the Guidebook are new, and many have been used extensively in Massachusetts. The Guidebook also provides a consistent reference book for designers and reviewers working in Winchendon. Recognizing that many best development practices are site-dependent, the Guidebook identifies a range of practices that are relevant to development and redevelopment projects on a variety of sites. Applicants for a Low Impact Development Permit, or who seek approval under Subdivision, Site Plan or other regulations, will find the Guidebook a helpful reference on low impact practices. Projects falling under the requirements of Winchendon s Low Impact Development Bylaw and Regulations, Subdivision, Site Plan or other regulations, are required to meet all of the specifications of those regulations. While this Guidebook accords with the above-referenced documents, it is not a substitute for meeting their procedures and requirements. Applicants who are required to meet the specifications of the Winchendon Low Impact Development Bylaw and Regulations must comply with the most recent version of the MA Department of Environmental Protection s Stormwater Management Standards and Handbook in the design and construction of stormwater management devices. The Guidebook is divided into five sections. Section I is a checklist for designers to help them determine which best development practices are likely to be applicable to their project. Project reviewers may also use this checklist to assess the project s conformity with the objectives set forth in the BDP Guidebook. Sections II through V discuss best development practices related to stormwater management, erosion and sedimentation control, landscape design, and site planning. These four sections describe the best development practices that are applicable in different situations and some technical details of the practices. References are provided for those who seek more information and design specifications for the various practices. Thank you for taking the time to read this Guidebook. With your participation the Town of Winchendon will become a model for attractive and environmentally responsible community development. We are grateful to the Town of Franklin, MA for providing their Best Development Practices Guide, upon which this Guide is based. Winchendon Best Development Practices Guidebook Page 1

4 I. Checklist for Designers The Checklist for Designers is a summary of the best development practices that this Guidebook recommends, and when they should be used. Prior to submitting an application for review, the applicant is encouraged to fill out this checklist to verify that he or she has complied with Winchendon s development objectives and planned the development or redevelopment site in a way that furthers the goals discussed below. The checklist may be submitted with the application. The Planning Board and their technical consultants may use the checklist to evaluate whether the application conforms with this Guidebook. Winchendon Best Development Practices Guidebook Page 2

5 Checklist for Designers Page 1 of 4 Stormwater Management GOALS and NEEDS addressed: 1. Protect local and regional wetlands and water bodies 2. Maximize groundwater recharge to retain a viable local groundwater supply and reduce flooding WINCHENDON OBJECTIVES: (A) All new development projects in Winchendon should meet the following three stormwater management performance objectives. All redevelopment projects should meet the objectives to the maximum feasible extent, and, if they fail to meet the objectives, should retrofit or expand existing stormwater management systems to improve existing conditions. (B) 1. Post-development peak discharge rates from the site should not exceed pre-development peak discharge rates from the site. 2. Annual groundwater recharge from the post-development site should approximate annual recharge from the pre-development site. 3. The stormwater management system should remove at least 80% of the average annual load of total suspended solids (TSS) from the post-development stormwater created on the developed site. Low impact, non-structural stormwater management systems should be used wherever site conditions allow, as outlined in the Guidebook. Drain pipe/catch basin systems may be used, in part or in whole, if the applicant can demonstrate that other systems are not feasible due to site conditions. BEST DEVELOPMENT PRACTICES One or more of the following may be used to meet the above objectives. Vegetated water quality swales (Dry & Wet) (recommended to collect runoff from roadways & parking lots) Vegetated filter strips (recommended to filter and infiltrate runoff from roadways, parking lots, and driveways; use with vegetated swales along roadsides and parking lots) Incorporated into Project? Constructed wetlands (preferred method for stormwater retention & pollutant removal) Bioretention cells (Rain Gardens) (recommended on residential lots and parking lot islands) Porous pavement (recommended in overflow parking and low-traffic areas) Tree box filter (recommended in urbanized locations) Green roofs (encouraged on flat commercial and industrial rooftops) Wet basins (less preferred method for stormwater retention & pollutant removal) Dry detention basins (less preferred, but may be used in series with other practices, such as constructed wetlands, to provide pre-treatment) Deep sump catch basin (discouraged, unless other stormwater collection and conveyance systems have been demonstrated not to be feasible due to site conditions) Have you evaluated the feasibility of other systems? Winchendon Best Development Practices Guidebook Page 3

6 Checklist for Designers Page 2 of 4 Erosion and Sedimentation Control GOALS and NEEDS addressed: 1. Minimize erosion 2. Prevent sedimentation of water bodies and its attendant environmental impacts WINCHENDON OBJECTIVES: (A) Any proposed project on a previously undeveloped site should accommodate the development program in a way that minimizes clearing and regrading, especially in areas of steep slopes, erosion-prone soils, or sensitive vegetation. For redevelopment projects, the site plan should concentrate development in previously-disturbed areas to the extent possible. (B) Proposed projects should submit and adhere to a construction management plan that addresses soil stabilization, sediment retention, perimeter protection, construction scheduling, traffic area stabilization and dust control. BEST DEVELOPMENT PRACTICES The applicant should comply with the following: Clearing and regrading have been minimized Development is focused in previously disturbed areas (for redevelopment projects) A construction management plan has been prepared Incorporated into Project? The construction management plan addresses: Soil stabilization (cover or stabilize erodible surfaces not in immediate use) Sediment retention (runoff interceptors and sediment traps/ponds) Perimeter protection (vegetated buffers or silt fences at the limit of work) Construction scheduling (minimize disturbed area at any given time) Traffic area stabilization (crushed rock or similar at construction vehicle entrance and parking areas) Dust control (plan for stabilizing dusty surfaces when necessary) Winchendon Best Development Practices Guidebook Page 4

7 Checklist for Designers Page 3 of 4 Landscape Design GOALS and NEEDS addressed: 1. Minimize demand for irrigation water 2. Maximize groundwater recharge from landscaped areas 3. Preserve native biodiversity by retaining habitat and defending against invasive species 4. Maximize the value to wildlife of human-managed landscapes WINCHENDON OBJECTIVES: (A) Site plans and landscape plans for all proposed projects should take appropriate steps, as outlined in the Guidebook, to minimize water use for irrigation and to allow for natural recharge of groundwater. (B) Landscape plans should follow the guidelines in the Guidebook for selecting species that are most appropriate to the site conditions. Native species and habitat-creating species should be used in all landscape plans to the maximum extent possible while still meeting the site s landscaping needs. Invasive species identified in this Guidebook should not be planted in Winchendon under any condition. BEST DEVELOPMENT PRACTICES The applicant should comply with the following: Clearing and regrading have been minimized (natural vegetation should be retained to the maximum extent possible, given the development program) Irrigation, if present, is water efficient (if an in-ground irrigation systems is proposed, it is a water efficient system with automatic sensors to prevent overwatering) Incorporated into Project? Landscaped areas retain water (gardens are mulched and designed for water infiltration) No invasive species are used (species from the invasive species list should not be used) Native and habitat-creating species are used (species from these lists have been incorporated into the landscape design whenever possible) Species are appropriate to the soil, site, and microclimate conditions (select appropriate species from the lists of salt-tolerant, urban-tolerant, wetland, moist-tolerant and droughttolerant species) Winchendon Best Development Practices Guidebook Page 5

8 Checklist for Designers Page 4 of 4 Site Planning GOALS and NEEDS addressed: 1. Protect Winchendon s natural environment, including habitat, water resources, and ecosystem services 2. Create a visually appealing community 3. Preserve the Town s historic and cultural heritage 4. Stabilize and increase property values 5. Encourage sustainable development WINCHENDON OBJECTIVES: Subdivision plans and site plans for all forms of development should adhere to the principles of environmental compatibility, aesthetic compatibility, and energy-efficient design. BEST DEVELOPMENT PRACTICES The site plan should address the following principles: Unique natural features have been preserved (the development program should avoid or else showcase significant natural features) Historic and cultural resources have been preserved (the development program should avoid or else showcase significant historic and cultural features) Incorporated into Project? Clearing, grading, and building placement consider viewsheds Cut and fill have been minimized Buildings blend into the natural topography Buildings are oriented to the sun and wind for maximum energy efficiency Vegetated protection from northwest (winter) winds is provided Deciduous species planted or retained close to the E, S and W building edges Winchendon Best Development Practices Guidebook Page 6

9 II. Stormwater Management a. Overview and Objectives The need for strong and innovative stormwater management practices is based on Winchendon s attempt to address several challenges: Winchendon has numerous streams, ponds and wetlands, and is at the headwaters of the Millers River, all of which are affected by polluted runoff. The Town s hilly topography contributes to stormwater runoff and flooding in developed areas. Better groundwater recharge will reduce flooding and help maintain base flows of rivers and streams. The U.S. Environmental Protection Agency (EPA) and MA Department of Environmental Protection (DEP) are creating more stringent requirements for communities to manage polluted runoff effectively. 1 (Notes and references are provided in the endnotes.) In order to attain a minimum level of stormwater management for development and redevelopment projects, Winchendon has adopted the following stormwater management performance objectives. 2 The performance objectives allow the design engineer to select one or more stormwater management systems that are most appropriate and cost-effective for the particular site. WINCHENDON OBJECTIVES: All new development projects in Winchendon should meet the following three stormwater management performance objectives. All redevelopment projects should meet the objectives and if they fail to meet the objectives, should retrofit or expand existing stormwater management systems to improve existing conditions. 1. Post-development peak discharge rates from the site should not exceed pre-development peak discharge rates from the site. 2. Annual groundwater recharge from the post-development site should approximate annual recharge from the pre-development site. 3. The stormwater management system should remove at least 80% of the average annual load of total suspended solids (TSS) from the post-development stormwater created on the developed site. There is a growing realization among water resource professionals that conventional systems of stormwater collection, conveyance, and end-of-the-pipe dry-basin detention are not sufficient to improve the water quality of surface water bodies. Therefore, the Town s preference is that stormwater be conveyed and treated in natural and vegetated systems such as vegetated swales, filter strips, constructed wetlands, and bioretention cells (rain gardens). While some of these practices may be new to the Town of Winchendon, they have been used successfully in other towns and states and have gained support from the EPA because of their generally superior performance in attenuating peak runoff rates, filtering WINCHENDON OBJECTIVES: Low impact, non-structural stormwater management systems should be used wherever site conditions allow, as outlined in the Guidebook. Drain pipe/catch basin systems may be used, in part or in whole, if the applicant can demonstrate that other systems are not feasible due to site conditions. Winchendon Best Development Practices Guidebook Page 7

10 pollutants, recharging groundwater, and allowing retention of the natural landscape. Recognizing that non-structural systems are not appropriate in all situations, the Guidebook also discusses other practices that can be used in certain situations. This section of the Guidebook discusses ten stormwater management practices that can be used, alone or in combination, to meet the performance objectives. Other systems not discussed in this guidebook may also be appropriate if the applicant can demonstrate their fulfillment of the above objectives. Table 2-1 provides a summary of the practices discussed in this chapter and when each practice is encouraged. Table 2-1: Use of Stormwater Management Practices in Winchendon Practice Vegetated Water Quality Swales (Dry & Wet) Winchendon s Objective Strongly encouraged Appropriate Uses Roadsides, parking lots Vegetated Filter Strips Strongly encouraged Roadsides, residential frontage areas, parking lots, perimeter protection Constructed Wetlands Strongly encouraged Commercial and industrial sites, office campuses, subdivisions Bioretention Cells (Rain Gardens) Strongly encouraged Residential lots, parking lot islands, urban & suburban locations; retrofits Porous Pavement Encouraged Parking overflow areas, driveways, walkways Tree Box Filter Encouraged Along streets and sidewalks in urbanized locations Green Roofs Encouraged Office/industrial buildings Wet Basins Neutral Subdivisions, office developments Dry Detention Basins Deep Sump Catch Basin Can be used in combination with other practices or when other systems are not practical due to site constraints Can be used when other systems are not practical due to site constraints All areas of development, if necessary All areas of development, if necessary There are several factors to consider when deciding on which practice(s) to implement in any given project. Among these factors are the space required, soils and slopes on site, depth to the water table, maintenance requirements, pollutant removal efficiencies, cost, and ability to meet Winchendon s stormwater performance objectives. Winchendon Best Development Practices Guidebook Page 8

11 b. Summary of Practices Table 2-2 provides a summary of design and site considerations for selected stormwater management systems. Table 2-2: Design and Site Considerations for Stormwater Practices 3 Space Required Soils Slope of Catchment Area Water Table and Bedrock Proximity to Building Foundations Max. Depth Maintenance Requirement Vegetated Water Quality Swales (Dry & Wet ) Bottom width: 2 ft. min. 8 ft. max. Permeable soils perform better, but wet swales can be used in less permeable soils Longitudinal slope of swale should be <5%. Slope should allow non-erosive flow rate The bottom of Dry Swales should be separated by 2 to 4 ft. from the seasonal high water table Min. 10 ft. downgradient from buildings & foundations recom d For Dry Swales maintain at least 1 foot of freeboard above volume expected for 10-year storm Low: routine landscape maint. Vegetated Filter Strips Minimum length, 25 feet. Minimum width should be 20% of the length of flow path or 8 ft., whichever is greater Permeable soils perform best. Avoid using in soils with high clay content or soils that cannot sustain grass cover Slope of contributing area should be between 2% and 6% Filter strips should be separated by 2 to 4 feet from the seasonal high water table Min. 10 ft. down-gradient from buildings & foundations recom d Constructed Wetlands 1% to 2% of watershed drainage area. Generally a watershed contributing area >10 to 25 acres is needed Preferred soils are mediumfine textured soils such as loams and silt loams Max. 15% for forested; 5% for shrubs/ herbs Water table should be at or near soil surface, or else a liner can be used to retain water Min. distance 10 ft. Bioretention Cells (Rain Gardens) Device size should be 5% to 7% of the area draining to it. Devices perform best with contributing area <5 acres Man-made soil mix is used consisting of sand, topsoil and compost. Underdrains allow for less permeable soils. Devices perform best on slopes of ~5%. Do not site on slopes >20% Separate bottom of device from groundwater to avoid groundwater contamination Min. 10 ft. downgradient from buildings & foundations recom d N/A 4 to 6 ft. 2 to 4 ft.. of soil media Low: routine landscape maint. Moderate; depends on sediment n rate Low: property owner can include in normal site landscape maint. Porous Pavement Not a factor Soils with permeability of at least 0.17 inches per hour Use on slopes <5% Minimum of 3 ft. vertical separation of bottom of storage layer from seasonal high groundwater level Not a factor 2.5 to 4 ft. for filtering and storage beds beneath surface Low: routine sweeping to reduce clogging of pores Tree Box Filter Not a factor Man-made soil mix is used. Underdrains allow for les permeable soils. Not a factor Not a factor Not a factor Wet Basins Min. pool surface area: 0.25 acres. Require drainage area from 20 acres to 1 sq. mi. Soils should allow permanent pool of water. Where soils are highly permeable, a liner may be used Maximum 15% Not a factor Min. distance of 10 ft N/A 8 ft. 3 to 6 ft. pool depth preferred Low: rake soil media to maintain filtration; check tree Moderate; depends on sediment n rate Dry Detention Basins 1 acre foot per 4 acres drainage area Highly impermeable soils may not allow water to drain Maximum 15% Bottom of basin should not intercept groundwater table Min. distance of 10 ft ft. Moderate: routine sediment removal Winchendon Best Development Practices Guidebook Page 9

12 Table 2-3 may be used for comparison with Winchendon s 80% TSS removal standard. Other systems not discussed in this Guidebook may also be used if the applicant can demonstrate that they meet Winchendon stormwater management performance standards detailed in the LID Regulations. If more than one practice is used to achieve the required 80% TSS removal, the removal efficiency rates must be multiplied together, not added. For example, if the first practice has a 60% TSS removal rate and the second practice has a 20% removal rate, a total of 68% of TSS would be removed. (60% of the total, plus 20% of the remaining 40% of TSS.) Table 2-3: TSS Removal Rates for Stormwater Management Practices 4 Practice Vegetated water quality swales (Dry & Wet) Design Rate for TSS Removal (use this number to calculate compliance with the 80% TSS removal requirement) 70%, when provided with pre-treatment device such as sediment forebay with check dam Range of Average TSS Removal Rates 60-80% Vegetated filter strip Constructed wetland a Bioretention cells (Rain Gardens) 10% if filter strip is 25 ft. wide; 45% if filter strip is 50 ft. wide 80% with sediment forebay for pretreatment 90% b with appropriate pre-treatment such as a vegetated filter strip 10-65%, depending on width 30-80% 65-90% Porous pavement 80% 65-90% Tree box filter 80%, presumed Insufficient data Wet basin a Dry detention basins c Deep sump catch basin 80%, when provided with pre-treatment sediment forebay MA DEP does not give credit for TSS removal 25%, only if used for pretreatment and designed as off-line systems 60-80% Insufficient data 25% w/cleanout a Must have sediment a forebay or pre-treatment. b If an underdrain is used, the water must be conveyed to a secondary treatment device such as a constructed wetland or a wet basin. c Post-treatment is required (e.g., by a constructed wetland or wet basin). Winchendon Best Development Practices Guidebook Page 10

13 c. Discussion of Practices Each of the ten stormwater management practices is discussed in more detail below. Most of the information has been taken from the MA DEP Stormwater Management Handbook. References in the endnotes provide additional information on the use, design, and construction of these practices. Persons who are required to meet the specifications of the Winchendon Low Impact Development Bylaw and Regulations must comply with the most recent version of the MA Department of Environmental Protection s Stormwater Management Standards and Handbook in the design and construction of stormwater management devices. Consult that document to meet regulatory requirements and for details about stormwater management practices. 1. Vegetated Water Quality Swales (Dry & Wet) Swales are earthen channels most commonly covered with a dense growth of grass or other vegetation, and are designed primarily to control water quantity and quality from a 10- year storm without causing erosion. 5 The design of vegetated swales has improved over the years, enabling engineers and hydrologists to implement them for a variety of different purposes. Depending on hydrological conditions and design, swales can be dry or wet: Figure 2-1 Dry water quality swale Dry swales Dry swales are designed to temporarily hold the water quality volume of a storm in a pool or series of pools created by permanent check dams at culverts or driveway crossings. The soil bed consists of native soils or highly permeable fill material, underlaid by an underdrain system.. Wet swales Wet swales also temporarily store and treat the required water quality volume. However, unlike dry swales, wet swales are constructed directly within existing soils and are not underlaid by a soil filter bed or underdrain system. Wet swales store the water quality volume within a series of cells within the channel, which may be formed by berms or check dams and may contain wetland vegetation. The pollutant removal mechanisms in wet swales are similar to those of stormwater wetlands, which rely on sedimentation, adsorption, and microbial breakdown. Design Considerations Swales are easily implemented on large lot residential sites (½ to 1 acre or larger), office and industrial campuses, roadways where right-of-way widths are adequate, and parking lot medians and edges (Figure Winchendon Best Development Practices Guidebook Page 11

14 2-1). Ideally, stormwater should flow from the impervious surface through a vegetated filter strip before entering the swales (see Figure 2-2). Typically, dry swales are used for low density residential projects or very small impervious areas and require soils that have infiltration rates of inches per hour. Wet swales, on the other hand, are convenient for treating highway runoff in low lying or flat terrain areas and need to be planted with water tolerant vegetation. Swales should be built at a gentle slope so that water flows at a relatively low velocity. The minimum slope should be ½%, while the maximum slope is based on velocity. Water velocity in the swale should generally not exceed 3 feet per second, which typically corresponds to a maximum slope of about 5%. If necessary, the swale may be steeper in places, provided that riprap or other stabilization is used to prevent scouring and erosion within the swale. The side slopes should be at a maximum of 3:1, and the length of the swale should be calculated to accommodate the entire calculated runoff volume from a 10-year storm. Regular maintenance such as mowing, sodding, and repair of eroded areas is necessary for swales. In addition, the accumulated sediment may need to be periodically removed, particularly during the construction and early site stabilization periods. Figure 2-2 Processes by which vegetated filters remove pollutants Winchendon Best Development Practices Guidebook Page 12

15 2. Vegetated Filter Strips Filter strips are typically bands of close-growing vegetation, placed between pollutant source areas and the receiving water body (either a natural water body or a constructed swale). To protect natural water bodies (e.g., streams or wetlands), filter strips should consist of natural buffer strips already existing on the site. Not only do filter strips protect sensitive areas such as wetlands, woodlands and erodible soils; they also reduce runoff impacts by trapping sediment and sediment-bound pollutants, provide some infiltration, and slow and disperse stormwater flow over a wide area. 6 Figure 2-2 illustrates some of the biochemical processes by which filter strips remove pollutants. 7 Figure 2-3 Vegetated filter strip Design Considerations Treatment of stormwater in filter strips is accomplished physically by a combination of filtration through the standing vegetation and infiltration into the underlying soils. In order to treat stormwater effectively, filter strips must be designed to function as overland flow systems where stormwater is evenly distributed. Because there is a high potential for short-circuiting of the filter strips and reduced pollutant removal, grading must be designed carefully to provide uniform flow into the filter strips, via devices such as a level spreader. If filter strips are wide enough and planted with appropriate plant species, they will provide wildlife habitat as well as visual amenities in the landscape. It is important to note that filter strips are usually implemented in combination with other stormwater management devices that specifically control stormwater volume. Finally, for a filter strip to be efficient, a minimum width of 15 to 30 feet is recommended. Upkeep of filter strips should be incorporated into routine landscape maintenance, which would include raking the filter strip, removing large trash or debris, and regularly cleaning up sediment. Winchendon Best Development Practices Guidebook Page 13

16 3. Constructed Wetlands Constructed wetlands (or stormwater wetlands) are shallow pools that create growing conditions suitable for marsh plants. These systems are designed to maximize pollutant removal through retention, settling, and uptake by wetland plants. 8 Stormwater wetlands serve several benefits simultaneously. The primary purpose of constructed wetlands is to improve water quality by removing sediment and pollutants. However, these wetlands can also Figure 2-4 Constructed stormwater wetland provide excellent habitat for wildlife and waterfowl. In general, a constructed wetland would be a suitable stormwater management practice for residential subdivisions and commercial developments. Constructed stormwater wetlands do not create additional, regulated water resource or buffer areas. Design Considerations Constructed wetlands must be designed with consideration to the size of the contributing watershed area, amount of baseflow, soil type, and available space. A contributing drainage area should be at least 10 acres. While the contributing watershed may be as small as 1 acre., the smaller the watershed area, the more difficult it is to create sufficient drainage and runoff to keep the wetland perpetually wet. Since wetlands need to maintain soil moisture throughout the year, it is important to have a dry-weather baseflow or a groundwater supply. The preferred soil types for constructed wetlands are less-permeable soils that have relatively small pores and are less prone to evaporation. The surface area of constructed wetlands should be at least 1% of the contributing drainage area, and the wetlands should have a length to width ratio of at least 2:1. 9 In order to increase the efficiency of the retention pond, a sediment forebay must be incorporated as a pretreatment device. As with all stormwater management practices, stormwater wetlands require ongoing maintenance to retain maximum effectiveness. However, several design features can decrease the amount of maintenance that a wetland needs. For example, a reverse-slope pipe or a weir outlet with a trash rack should be used to prevent clogging of the outlet; orifices should have diameters no less than 3 ; and direct maintenance access should be provided to the forebay to allow for sediment removal. Selection of plant species is one of the most important parts of creating a stormwater wetland, as the plants are largely responsible for the pollutant and sediment retention and uptake. Please refer to Figure for a sample wetland layout and Section IV for a list of plant species suitable for planting in constructed wetlands. Winchendon Best Development Practices Guidebook Page 14

17 Figure 2-5 Example of constructed stormwater wetland, shallow marsh type Winchendon Best Development Practices Guidebook Page 15

18 4. Bioretention Cells (Rain Gardens) Bioretention is a technique that uses soils, plants, and microbes to treat stormwater before it is infiltrated and/or discharged. Bioretention cells (also called rain gardens in residential applications) are shallow depressions filled with sandy soil topped with a thick layer of mulch and planted with dense native vegetation. Stormwater runoff is directed into the cell via piped or sheet flow. The runoff percolates through the soil media that acts as a filter. There are two types of bioretention cells: those that are designed solely as an organic filter filtering bioretention areas and those configured to recharge groundwater in addition to acting as a filter exfiltrating bioretention areas. A filtering bioretention area includes an impermeable liner and underdrain that Figure 2-6 Rain garden on a residential lot intercepts the runoff before it reaches the water table so that it may be conveyed to a discharge outlet, other treatment practices, or the municipal storm drain system. An exfiltrating bioretention area has an underdrain that is designed to enhance exfiltration of runoff into the groundwater. Runoff is conveyed to the treatment area, which consists of a grass buffer strip, sand bed, ponding area, organic layer or mulch layer, planting soil, and plants (see Figure ). Runoff passes first over or through a sand bed, which slows the runoff and distributes it evenly along the ponding area. The ponding area is made up of a surface organic layer, ground cover and the underlying planting soil. The ponding area is graded so that there is a depression in the middle where water remains until it infiltrates or evaporates. The depression should be designed to hold 6 to 8 inches of water. 12 An overflow structure should be provided for situations where the ponding area is not sufficient. Bioretention cells can be used in both residential and commercial projects. In residential areas, rain gardens are used to retain and infiltrate stormwater locally so that it does not need to be conveyed and treated by means of a more extensive stormwater management system. Each residential lot would typically have one or more rain gardens that receive stormwater from the roof and driveway and infiltrate it to the ground. Homeowners are responsible for maintaining the rain gardens on their property, just as they would maintain their garden. In commercial projects, bioretention cells are installed as depressed islands in the parking lot. Stormwater is directed to these islands, where it is treated and infiltrates into the ground. Figure 2-7 Bioretention cell at a commercial site Photo courtesy Berkshire Design Group Winchendon Best Development Practices Guidebook Page 16

19 Figure 2-8 Example of bioretention Design Considerations The design of bioretention cells must consider the site area, slope, soils, groundwater, and maintenance needs. Bioretention cells should be designed to occupy 5 to 7 percent of the drainage area. Bioretention cells receiving parking lot runoff are recommended to have minimum dimensions of approximately 15 feet by 40 feet. Rain gardens on individual house lots can be much smaller, on the order of 10 feet by 15 feet. The site should have shallow slopes (approximately 5% or less) so that water flow is guaranteed but velocity is not too high. An underdrain should be used in situations where soils are less permeable and there is a concern about the cell backing up or flooding. (Bioretention cells may be constructed by importing more permeable soils for the cell itself, in combination with an underdrain which prevents water from ponding above a less permeable natural soil layer.) An underdrain should also be used where the water table is close to the surface and there is a concern about groundwater pollution. In other situations, an underdrain generally is not needed. Bioretention cells need to be maintained regularly to ensure the presence of mulch and good soil, attend to any diseased or dead plants, and remove collected sediment, litter and debris. Winchendon Best Development Practices Guidebook Page 17

20 5. Porous Pavement Porous pavement typically consists of a permeable surface with an underlying crushed/broken stone reservoir to temporarily store runoff before it infiltrates into the ground. The main purposes of this application are to reduce the amount of stormwater runoff from paved areas and to infiltrate stormwater into the underlying soils. By reducing the amount of stormwater runoff, pervious paving surfaces reduce the cost of stormwater management. Porous pavement may be used for walkways, patios, plazas, driveways, parking stalls, and overflow parking areas. Figure 2-9 Porous asphalt Porous pavement that is now available includes porous asphalt, pervious concrete and grass pavers. Porous asphalt and pervious concrete appear to be the same as traditional pavement from the surface, but incorporate void spaces to allow infiltration. These systems have been applied successfully in New England locations where soils consist of well-drained sands and gravels that allow the pores to drain rapidly. Porous pavement can be successfully installed in cold climates as long as the design includes features to reduce frost heaving. Grass pavers (Figure 2-10) consist of concrete interlocking blocks or a synthetic fibrous gridded system with open areas designed to allow grass to grow. The design should allow for infiltration into the underlying soils so that stormwater does not pond near the surface. Figure 2-10 Grass pavers grass pavers are recommended. Design Considerations Grass pavers are most suitable for low-traffic areas such as the overflow areas of commercial or office parking lots, residential driveways, and service areas that will be subject to light traffic. Winchendon recommends their use in these situations. If these systems are used, stormwater calculations should account for the reduced amount of runoff generated by areas with grass pavers. In general, grass paver systems should not be salted in the winter because this will threaten the viability of the plants. However, salting will not typically be required in the low-traffic areas where Winchendon Best Development Practices Guidebook Page 18

21 6. Tree Box Filter A tree box filters is a small bioretention system that provides effective water quality treatment for stormwater in urban settings where space is limited. It consists of a concrete barrel, usually with an open bottom, that contains a porous soil mix, a crushed gravel underdrain and a tree or shrub. Stormwater is directed from the sidewalk and street into the tree box where sediment and other pollutants are filtered through the soil mix. Treated stormwater overflow from the tree box filter is directed to the underdrain and may then enter a storm drain or other stormwater treatment device. Figure 2-11 Tree box filter Tree box filters are effective pretreatment devices which also reduce stormwater volume and rate of runoff. They can be useful as retrofits in urban settings and at redevelopment sites. Tree boxes, however, treat relatively small volumes of water. The contributing area for a tree box should not exceed 0.1 acre. Winchendon Best Development Practices Guidebook Page 19

22 7. Green Roofs Figure 2-12 Green roof Green roofs are a permanent rooftop planting system containing live plants in a lightweight engineered soil medium designed to retain precipitation where the water is taken up by plants and transpired into the air. As a result, much less water runs off the roof compared to conventional rooftops. Green roofs provide several benefits such as reducing stormwater runoff volume and pollutant load, increasing the energy efficiency of buildings, improving air quality by removing particles in the air and by photosynthesis, and increasing the aesthetic value of the area. Green roofs can be built on almost any rooftop, from a residential building to a commercial or industrial building. Figure 2-12 shows a typical application. 13 The Town of Winchendon recommends that green roofs be used in large commercial, office, and institutional buildings that have flat or gently sloping roofs. Green roofs with a restricted variety of plants (resistant to frost, wind and drought: sedum, herbs and grasses) require minimal long-term maintenance. During the initial stage some watering might be required; however, within six months the plants are usually able to sustain themselves. Since the soil layer is not deep, it will not support tall vertical growth or large plants; therefore, cutting or mowing is not required. Load reserves of at least 15 pounds per square feet beyond snow load requirements are needed to install a roof garden. 14 If properly built over a suitable roofing membrane, roof gardens do not present a leaking problem. Winchendon Best Development Practices Guidebook Page 20

23 8. Wet Basins Wet basins are constructed to have a permanent pool of water to treat stormwater. The pool allows settling of sediments, removal of soluble pollutants by algal uptake, and some groundwater recharge as shown in Figure The basins are designed to include additional storage capacity to control peak discharge rates. The primary component of a wet basin is a deep, permanent pool, and the basin must also include a shallow marsh or a sediment forebay to increase sediment and nutrient removal. 16 In general, the Town of Winchendon does not recommend that Figure 2-13 Wet basin wet basins be used as the primary means of attenuating peak runoff rates or removing pollutants. Constructed wetlands are generally a preferred system since they have greater pollutant uptake functions. However, wet basins may be used in subdivisions as well as commercial and industrial areas when other stormwater management systems are not feasible or sufficient because of site conditions or the nature of the development program. Wet basins may also be used in series with constructed wetlands as a sediment trap, particularly during construction. Design Considerations Wet basins must drain a sufficiently large area to maintain a permanent pool of water. The minimum recommended area is 20 acres, assuming impervious surface percentages typical of suburban developments. 17 Within the watershed area that drains to the wet basin, the slopes and the stormwater conveyance system must result in a metered flow of stormwater that does not flood the basin all at once. The use of filter strips and swales can help slow and infiltrate water on its way to the basin. Wet basins can be constructed in a wide range of soil types. However, when native soils have a rapid percolation rate, soils should be compacted or supplemented sufficiently so that the pond does not dry up during the dry season. The soils should retain sufficient infiltration potential so that the pond also continues to play a role in groundwater recharge. In order to increase the efficiency of the wet basin, a sediment forebay must be incorporated as a pretreatment device. As with constructed wetlands, wet basins should use non-clogging outlets and large orifices (not less than 3 inches in diameter), and should provide easy dredging access to reduce long-term maintenance requirements and difficulties. Winchendon Best Development Practices Guidebook Page 21

24 9. Dry Detention Basins Dry detention basins are depressed areas whose outlets have been designed to detain stormwater runoff for some minimum time to allow particles and associated pollutants to settle. Since the aim of dry detention basins is mainly to control flooding and remove sediments, they do not need to have a permanent pool and therefore can be dry during non-flood conditions. Typically, they are used as a primary treatment in conjunction with other stormwater management systems such as wet basins or constructed wetlands. Figure 2-14 Dry detention basin The Town of Winchendon discourages the use of detention basins as the primary means of flood control. Instead, dry water quality swales, vegetated filter strips and bioretention should be used whenever possible to attenuate peak runoff rates. When site characteristics do not allow use of such practices, however, detention basins may be considered as part of the stormwater management system. In addition, detention basins may be used as a pre-treatment device to settle out particulates prior to discharge to a constructed wetland or wet basin, where additional treatment and infiltration will occur. Design Considerations Dry detention basins are most practical for use on sites of at least 10 acres, which allows for the use of larger outlet orifices that are less likely to clog. Detention basins can be used on sites with slopes up to 15 percent. To provide drainage, make the minimum slope of the bottom 2 percent. If soils on site are relatively impermeable (such as Soil Group D), a dry detention basin may experience problems with standing water. On the other hand, if the soils are highly permeable, such as well-drained sandy and gravely soils (Soil Group A), it will be difficult to establish a shallow marsh component in the basin. Dry detention basins should be designed with sediment forebays which allow sediment to be trapped prior to entering the detention basin. This feature also reduces maintenance requirements for the detention basin. As with constructed wetlands, detention basins should use non-clogging outlets and large orifices (not less than 3 inches in diameter), and should provide easy dredging access to reduce long-term maintenance requirements and difficulties. Any dry detention basin in the Town of Winchendon must provide landscaping and planting to minimize its visual impacts. The plants selected for the ponding area should be able to withstand both wet and dry periods. Along the perimeter of the basin, however, the plants should be adapted to dry conditions and should create a visual vegetated buffer. Winchendon Best Development Practices Guidebook Page 22

25 Figure 2-15 Dry detention basin schematic Winchendon Best Development Practices Guidebook Page 23

26 10. Deep Sump Catch Basin Figure 2-16 Deep sump catch basin inlet Catch basins are storm drains that capture and roughly filter stormwater through a grate or curb inlet and capture sediment, debris and associated pollutants in a deep sump (Figure ). In most cases a hood is also included to separate oil and grease from the stormwater. The essential function of a catch basin is to act as a pretreatment device for other structures incorporated into a storm sewer system. The performance of a catch basin in removing sediment and pollutants will depend greatly on the size of the drainage area, the size of the sump, and the amount of maintenance it receives. Although catch basins are currently used in virtually all circumstances, they typically cannot remove pollutants as well as most of the other practices described in this section, and they require frequent maintenance. Catch basins should be used only when other practices discussed in this section prove unfeasible. When catch basins are used, the Town will consider it preferable if they discharge individually or in pairs to nearby swales, constructed wetlands, or bioretention cells, rather than carrying runoff further to a larger retention/infiltration system. Lengthy catch basin-piped drain-manhole networks are discouraged. In general, the goal is to use vegetated, low-velocity channels to hold and infiltrate stormwater locally, not to efficiently capture and deliver stormwater to watercourses. Design Considerations The contributing drainage area to any deep sump catch basin should not exceed 1/4 acre of impervious cover. Catch basins should have sumps that are at least four feet deep. The inlet openings must not allow flows greater than 3 cfs to enter the deep sujp catch basin. Flow from the catch basin/drain pipe system should be directed to another stormwater management device, such as a constructed wetland, for further treatment. Catch basin inlets should be cleaned regularly (at least twice a year) and after large storms. Removed sediment should be disposed of in accordance with applicable local, state and federal guidelines and regulations. Winchendon Best Development Practices Guidebook Page 24

27 III. Erosion and Sedimentation Control Erosion and sedimentation control practices should be incorporated into the planning, construction, and operation of any project in Winchendon. Specific measures should be presented for review prior to construction. For more detailed information please consult the MA DEP Erosion and Sediment Control Guidelines. a. Site Planning The most important erosion control practice is to minimize clearing and regrading, as discussed in Section V, Site Planning. WINCHENDON OBJECTIVES: Any proposed project on a previously undeveloped site should accommodate the development process in a way that minimizes clearing and regrading, especially in areas of steep slopes, erosion-prone soils, or sensitive vegetation. For redevelopment projects, the site plan should concentrate development in previously-disturbed areas to the extent possible. The initial step to control erosion and sedimentation lies in developing a plan that is appropriate to the site features including topography, soils, drainage ways, and natural vegetation. The site planning process should begin with a thorough evaluation of sensitive areas requiring protection as well as less sensitive areas suitable for development. The site plan should delineate a limit of work that limits clearing and regrading and protects the most sensitive areas, based on the criteria in Table 3-1. For example, in residential subdivisions, native vegetation should be retained on individual house lots to the extent possible, rather than creating larger lawns. Table 3-1: Guidelines for identifying sensitive site features 19 Topography Drainage Soils Natural vegetation Slopes that are steeper and/or longer typically create more erosion. Slopes that exceed the following thresholds are likely to be sensitive and erosionprone: Slopes of 5-7% longer than 300 feet Slopes of 7-15% longer than 150 feet Slopes of more than 15% longer than 75 feet Where possible, retain natural drainage ways and depressions and utilize for stormwater conveyance Consider factors such as erodibility, permeability, depth to water table and bedrock, and soils with shrink/swell potential or slippage tendencies. The most erodible soils contain high proportions of silt and very fine sand. The presence of clay or organic matter tends to decrease erodibility. This is the most important factor in preventing erosion. Vegetated buffers filter runoff, decrease runoff velocity, and increase infiltration capacity. Winchendon Best Development Practices Guidebook Page 25

28 b. Construction Period Impacts Prior to the commencement of construction, the limit of clearing and limit of work identified on the site plan should be suitably marked with survey tape or plastic fences. These markers are in addition to any fences that the Conservation Commission may require for sensitive areas such as wetlands, streams and their buffers. Construction activities and construction traffic should be limited to the area identified on the site plan, and no stockpiling of materials, soils, or debris or other activity should occur outside of the limit of work. WINCHENDON OBJECTIVES: Every proposed project should adhere to a construction management plan that addresses soil stabilization, sediment retention, perimeter protection, construction scheduling, traffic area stabilization and dust control. Winchendon Best Development Practices Guidebook Page 26

29 1. Soil Stabilization The builder/developer should create a plan for cover and/or stabilization of erodible surfaces that are not the immediate focus of construction activity. The Town's objective is to minimize soil erosion and sedimentation in the Town s water bodies and storm sewer system. Cover measures should be implemented on areas that have already been disturbed but will not be worked on during the next 7 days during dry conditions or next 2 days during wet conditions. Cover methods may include, but are not limited to, the use of mulch, erosion control nets and blankets, plastic covering, seeding and sodding. These are described in the following paragraphs. Mulching is generally considered to be a suitable short-term protective measure. The main purpose of mulching is to protect the site from erosion by stabilizing soils and reducing stormwater runoff velocity. Mulch can also enhance plant establishment by conserving moisture, holding fertilizer, seed and topsoil in place, and moderating soil temperature. The most commonly used mulches include straw, wood fiber or cellulose, compost and wood chips. The effectiveness of mulching depends on site characteristics and maintenance: if the site is prone to high winds or has steep slopes, additional steps should be taken to anchor the mulch, such as planting vegetation or providing netting or blanketing. The thickness of the cover should be maintained at all times and any area that has eroded should be re-mulched and anchored until it has been stabilized. Similar to mulching, plastic covering is also acceptable as a short-term protective measure. This technique simply involves covering the area of concern with a plastic sheet and using tires or sandbags to weight the plastic down. Plastic covering is generally used on cut and fill slopes and stockpiles. Plastic covering should not be used if there is a sensitive area located downslope, because of the rapid runoff created by the plastic covering. Although this is a fairly easy technique to apply, it requires careful maintenance. The plastic cover can easily be torn or damaged by the sun and can clog drainage systems if not removed properly. Therefore, regular maintenance should be provided to ensure that the plastic is undamaged at all times and fully removed after it is no longer needed. Figure 3-1 Geotextiles used for stabilizing a hill Seeding, sodding and erosion nets and blankets are usually more appropriate as longterm solutions for areas that will remain unworked for months. Section IV lists recommended species to plant for erosion control purposes. A well-designed landscaping plan can easily incorporate areas of planting for permanent erosion control. If the area must be stabilized immediately, then the use of sodding is more appropriate since it can provide immediate erosion protection. Sodding is appropriate for use on residential or commercial lawns, steeply-sloped areas, waterways and channels carrying intermittent flow, and areas around drop inlets that require stabilization. 20 Sod maintenance is essential during the establishment period. Sod should be provided with adequate moisture and fertilizer. If the sod does not root and stay healthy, it should be replaced by new sodding or a different technique. Winchendon Best Development Practices Guidebook Page 27

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